1 . each completed run of census triggers data that increments snapshot counts.. so yes

2 . the in game database only keeps track of the last seen level for each character seen, so if you only upload after multiple levels of the toon.. there will be jumps in character level in the website database. Assuming no other players are doing census runs and uploading during the time frame between your leveling.

If you want to accurately record your characters level (recording each level increase) you need to upload the data after each level ding for your character.

Yes if you are actively playing a very low level character on a very high population realm you can get multiple levels (in the range of 1 - 5 for example) while the whole currently logged in realm faction is being counted in one snapshot run.